What is Address $08000000? Coprocessor Slot Expansion?

[selco]

Hello,
in the A1200 Kickstart (3.1) in expansion.library function rt_Init after checking (and calling if valid) ColdCapture I found some code I don't understand. It deals with address $8000000 and seems to check it's infuence to address Null. A few bytes later similar things happen with address $8000004.

What is the purpose of this code? What does it check for? I found in the A3000 memory map:$0800 0000 - $0FFF FFFF Coprocessor Slot Expansion

Is that the math coprocessor?

Is there some documentation I could read?

;Addr f8395e (A1200, OS3.1)     
        
SUBA.L  A2,A2          ; A2=0
MOVEA.L #$8000000,A0   ; $0800 0000 - $0FFF FFFF Coprocessor Slot Expansion (A3000)
MOVE.L  (A2),D2        ; D2 = contents of Address Null
NOT.L   (A0)           ; invert contents of addr $0800.0000
MOVE.L  (A2),D1        ; contents of addr 0 to D1
MOVE.L  D2,(A2)        ; write contents of addr Null to Null again
CMP.L   D2,D1          ; is it the same or dit NOT ($800000) change content of addr Null?
BNE.B   lbC00020C

best regards

[Toni Wilen]

$08000000-$0fffffff is reserved for CPU accelerator board RAM expansion. (also used by some A1200 boards too, it is not limited to A3000/A4000)

Check makes sure address $08000000 exists, for example $08000000 becomes $00000000 if CPU only has 24-bit address space (detecting between 68020 and 68EC020) or if not all address lines are connected. You don't want to overwrite execbase and exception vectors when testing memory

[selco]

Ahh, thanks!